1. Field of the Invention
The invention relates to a power-supply circuit for charging a battery with a charging current, comprising, between a first and a second power supply terminal, a first series arrangement of a primary winding of a transformer, a first transistor switch having a control input, a first resistor, and a second series arrangement of a secondary winding of the transformer and a first diode, the second series arrangement comprising terminals for the connection of a battery, a positive-feedback path between the node of the secondary winding and the first diode and the control input of the first transistor switch comprising the series arrangement of a second resistor and a first capacitor, the terminal of the first capacitor which is remote from the second resistor being coupled to the cathode of a zener diode, first switching means for turning off the first transistor switch at a specific voltage across the first resistor, the first switching means comprising a first input coupled to that end of the first resistor which is nearest the first transistor switch, a second input coupled to that end of the first resistor remote from the first transistor switch, and an output which is coupled to the control input of the first transistor switch, and second switching means for changing over from the charging current to a trickle-charging current above a specific threshold voltage.
Such a circuit can be employed for charging a battery and/or powering a load with an input voltage which may be either a rectified alternating voltage or a direct voltage. In particular, such a circuit is suitable for use in a shaver for charging the battery and/or powering the motor.
2. Description of the Related Art
A power supply circuit of the type defined in the opening paragraph is known from European Patent Application No. 0,226,253. In this circuit a current flows through the primary winding during the so-called forward interval, as a result of which energy is stored in the transformer. The primary current is converted into a voltage across a resistor. When a specific value of this voltage is reached, the first switching means turn off the first transistor switch, causing the primary current to be interrupted. The stored energy is then supplied to the battery as a linearly decreasing charging current via the secondary winding and the first diode during the so-called flyback interval. After the flyback the next forward interval is started by the positive feedback between the secondary winding and the control input of the first switch. In this way the battery can be charged comparatively rapidly with a comparatively large current.
In order to prevent the battery from being damaged by overcharging, the known power supply circuit comprises second switching means which turns off the power supply circuit via the first switching means if the battery voltage exceeds a threshold value and which subsequently renders the circuit operative at the instant at which the battery voltage has dropped below a specific second value. As a result of this, the circuit is switched from charging to trickle charging when the threshold value is exceeded first time.
However, the second switching means in the prior art power supply circuit comprises a comparatively large number of components, which renders the circuit more susceptible to component tolerances.